Theresa May is gearing up for a major speech outlining her plans for Brexit

Foreign Secretary Boris Johnson and Brexit Secretary David Davis are contributing to the content of the address, which is expected later in January.

Mrs May will reportedly say Britain will pull out of the single market if the European Union fails to make concessions on freedom of movement, although Downing Street sources insisted the claims were speculation.

She moved swiftly to draw a line under the row sparked by the surprise resignation of Sir Ivan, the disgruntled top UK civil servant, by agreeing his replacement within 36 hours of his notice to quit.

Sir Tim Barrow has been chosen to be the UK’s new ambassador to the EU

But Ukip’s former leader Nigel Farage said: “Good to see that the Government have replaced a knighted career diplomat with … a knighted career diplomat.”

Speaking the morning Tom Fletcher, a former UK ambassador to Lebanon and former foreign policy adviser to three UK prime ministers, welcomed Sir Tim’s appointment.

He told BBC Radio 4’s Today programme: “It’s the toughest negotiation in our lifetimes and I think he is up to it.

“I have seen him in Brussels. He knows the corridors, he knows the characters.

“But actually more importantly I saw him in Moscow where he was incredibly resilient as ambassador there, dealing with (Vladimir) Putin in a very testing time in our relationship and Tim had a reputation of being bulletproof out there.”